Fix some comments.

- Remove comment about blanket implementation of `FallibleTypeFolder` --
  it does not exist. (It probably used to.)
- Fix some incorrect name references.
- Fix minor grammatical errors.
- Fix stale comment on `visit_region` -- it was a no-op once, but no
  longer.

LLM disclosure: Claude Code identified these problems with comments when
I asked it to review `fold.rs` and `visit.rs`. I verified the
correctness of the problems and made the changes by hand.

Author: nnethercote

compiler/rustc_type_ir/src/flags.rs

@@ -139,7 +139,7 @@ bitflags::bitflags! {
         /// Does this type have any coroutines in it?
         const HAS_TY_CORO                 = 1 << 25;
 
-        /// Does this have have a `Bound(BoundVarIndexKind::Canonical, _)`?
+        /// Does this have a `Bound(BoundVarIndexKind::Canonical, _)`?
         const HAS_CANONICAL_BOUND         = 1 << 26;
     }
 }

compiler/rustc_type_ir/src/fold.rs

@@ -121,10 +121,6 @@ pub trait TypeSuperFoldable<I: Interner>: TypeFoldable<I> {
 /// default that does an "identity" fold. Implementations of these methods
 /// often fall back to a `super_fold_with` method if the primary argument
 /// doesn't satisfy a particular condition.
-///
-/// A blanket implementation of [`FallibleTypeFolder`] will defer to
-/// the infallible methods of this trait to ensure that the two APIs
-/// are coherent.
 pub trait TypeFolder<I: Interner>: Sized {
     fn cx(&self) -> I;
 
@@ -477,10 +473,10 @@ where
 /// Folds over the substructure of a type, visiting its component
 /// types and all regions that occur *free* within it.
 ///
-/// That is, function pointer types and trait object can introduce
-/// new bound regions which are not visited by this visitors as
+/// That is, function pointer types and trait objects can introduce
+/// new bound regions which are not visited by this visitor as
 /// they are not free; only regions that occur free will be
-/// visited by `fld_r`.
+/// visited by `fold_region_fn`.
 pub struct RegionFolder<I, F> {
     cx: I,
 
@@ -489,7 +485,7 @@ pub struct RegionFolder<I, F> {
     /// binder, it is incremented (via `shift_in`).
     current_index: ty::DebruijnIndex,
 
-    /// Callback invokes for each free region. The `DebruijnIndex`
+    /// Callback invoked for each free region. The `DebruijnIndex`
     /// points to the binder *just outside* the ones we have passed
     /// through.
     fold_region_fn: F,

compiler/rustc_type_ir/src/visit.rs

@@ -67,7 +67,7 @@ pub trait TypeVisitable<I: Interner>: fmt::Debug {
     /// each field/element.
     ///
     /// For types of interest (such as `Ty`), the implementation of this method
-    /// that calls a visitor method specifically for that type (such as
+    /// calls a visitor method specifically for that type (such as
     /// `V::visit_ty`). This is where control transfers from `TypeVisitable` to
     /// `TypeVisitor`.
     fn visit_with<V: TypeVisitor<I>>(&self, visitor: &mut V) -> V::Result;
@@ -102,8 +102,8 @@ pub trait TypeVisitor<I: Interner>: Sized {
         t.super_visit_with(self)
     }
 
-    // The default region visitor is a no-op because `Region` is non-recursive
-    // and has no `super_visit_with` method to call.
+    // `Region` is non-recursive so the default region visitor has no
+    // `super_visit_with` method to call.
     fn visit_region(&mut self, r: I::Region) -> Self::Result {
         if let ty::ReError(guar) = r.kind() {
             self.visit_error(guar)
@@ -252,7 +252,7 @@ pub trait TypeVisitableExt<I: Interner>: TypeVisitable<I> {
     }
 
     /// Return `true` if this type has regions that are not a part of the type.
-    /// For example, `for<'a> fn(&'a i32)` return `false`, while `fn(&'a i32)`
+    /// For example, `for<'a> fn(&'a i32)` returns `false`, while `fn(&'a i32)`
     /// would return `true`. The latter can occur when traversing through the
     /// former.
     ///
@@ -571,7 +571,7 @@ impl<I: Interner> TypeVisitor<I> for HasEscapingVarsVisitor {
         // `outer_index`, that means that `ct` contains some content
         // bound at `outer_index` or above (because
         // `outer_exclusive_binder` is always 1 higher than the
-        // content in `t`). Therefore, `t` has some escaping vars.
+        // content in `ct`). Therefore, `ct` has some escaping vars.
         if ct.outer_exclusive_binder() > self.outer_index {
             ControlFlow::Break(FoundEscapingVars)
         } else {